The problem with taking this sort of mathematical/statistical approach, is it isn't granular; it's sweeping. But it has the veneer of authenticity. It doesn't reflect the experience of individuals. A city may not be overpopulated but small, often poor, areas of it are likely to be. It is the experience of these people (both indigenous and immigrant) that is detrimentally affected by poor (or poorly executed) immigration policy. This is where the ill-feeling and tension arises. If it is not addressed and those local people who are suffering are simply shown a graph and accused of racism, the tension increases. In the UK, immigrants are a net contributor to the Exchequer. Which is fantastic. But it doesn't help the individual who is experiencing anxiety or difficulty as a result of immigration in their immediate environment.
I'm pro-immigration. But I'm also keen to respect and address the concerns of native people. Surely, we should all be striving to achieve a positive immigration experience.
Oh, enough. The most diverse places don't have these issues. This phobia is occurring mostly in places that are homogeneous. It's just classic racial resentment presented by mostly uneducated people whose identities are wrapped up in petty mythologies about themselves. It's not complex.
They are not experiencing sweeping demographic changes in their own neighborhoods. Besides, change occurs, populations move in, and some of them don't hold your values. You don't own the future.
You are free to move. Brexit is not going to do anything to change demographics, or immigration. It was sold to the plebs as the solution, but that was just cynical marketing. Once they realize nothing is going change in that regard, they will move on to their next self-defeating big idea.
The world has been through this before, and it always ends the same way. People are not complex, they are base, and easily defined. Fear is not an excuse. It's just being used a rationalization for a racial, and cultural resentment that was always there.
But as someone else said, as an American, I almost get a cruel satisfaction from watching England cripple itself. It has been America's lapdog since the end of WWII, and now it will be even more dependent on it. It's just trading dependencies. England already has privileged trade status with America. It can't get much sweeter.
I think that's what it mostly comes down to. England's national self-image is so tattered that it's now resorting to what you saw happen in Germany pre-Hitler. Hitler used nationalism to prick savage tendencies that gave him even more power to create his own agenda, and the public went along with anything he devised after that point; as long as he told them how great they were. They took a single issue, and traded in all others for the pleasure of scapegoating their failures.
Virulent nationalism is almost always a response to a society's low self opinion in an ever changing world. Somebody else must pay for making us feel small.
Stop trying to spice it with a sense of virtue, and reasonable anxiety. There is nothing reasonable about it. Most people didn't even bother to examine the details. It was a temper tantrum.